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surface to the resistance of the water while, on' the forward stroke, each moves freely and, sharp edged as they are, passes through the water like a feathered oar, with the least possible friction. The efficiency of the back stroke is increased by a fringe of long flattened setae which open like a fan, giving to the limb a scoop like form, and progressively increasing the resistance to the completion of the stroke and then, as the leg is drawn forwards, falling back smoothly to cause the minimum of friction. In quiet waters or slow currents the insects maintain their position by gentle sculling with the middle pair of legs only; when alarmed or in strong currents both pairs are used. Their efficiency is remarkable, Few creatures can swim morę than five times their own length per second; a fish, for example, rarely if ever covers a distance of morę than four times its own length in one second. DALGLEISH has computed that Gyrinus sp. can move twelve times its own length in a second and HATCH estimates that Dineutus discolor can cover between 16 and 17 times its own length in that period of time. Orecłochilus is a faster swimmer than either. What speed it attains I do not know but there can be little doubt that, rela-tive to its size, it moves faster than any racing motor boat yet produced.

The antennae are unusually thick and compact with a sca-le-like outgrowth near the base. The short thick antenna is car-ried out of the water and is prevented from being wetted, when the insect is swimming on the surface, by the scalę, In the second segment lies a sense organ believed, by EGGERS,¹) to detect differences in surface tension and thus enable the animal to be aware of solid objects projecting above the surface when it approaches them. That some such sense exists is probable; nocturnal species would otherwise be in hard case.

The compound eyes are divided into two pairs; one pair, ventral, serving to detect submerged objects; the other, dorsal and set well above the water on the top of the head, surveys the whole expanse of heaven. Diurnal forms hunt by sight and have keen yision. They can, as anyone who attempts to collect

1

Eggers F,₍ 1926. Die mutmassliche Funktion des Johnstonschen Sinnesorgan bei Gyrinus. Zool. Anzeiger, Bd. 68, pp, 184—192,